Suggest Treatment For Severe Itchy Rashes On The Neck

I am a 66 year old woman. Lately my neck has been itchy and I have a rash, especially at night. I don t see it anywhere else on my body. It seems as though it is tactile, but I cannot narrow it down to a garment or detergent (same detergent for 20 years). I swim in a chlorine/salt water pool at least four or five days a week, and it did start after I began swimming there, but the rash isn t anywhere else on my body which makes me suspicious. I am overweight (I though I should mention this0 although I swim, as I said, four or five days a week, walk at a good clip three miles several days a week and work out with a trainer/physical therapist one day per week. Any ideas?